O'Mahony made his debut for Munster A on 27 November 2009, replacing Simon Zebo in a British and Irish Cup game against Bristol and scoring a try.

[5] O'Mahony made his debut for the senior Munster team on 19 April 2013, starting against Dragons in a Pro12 fixture and scoring a try.

[11] On 1 May 2017, it was announced that O'Mahony would miss the remainder of the 2016–17 season after suffering a fractured fibula in Munster's win against Benetton on 29 April 2017.

[12] He made his return from the injury on 19 January 2018, coming off the bench for Munster A in their 27–0 British and Irish Cup win against Ospreys Premiership Select.

[14] O'Mahony made his debut for Ireland Under-20 on 14 February 2009, coming off the bench against Italy Under-20 and scoring a try in the 2009 Under-20 Six Nations Championship.